We Americans have our signature sandwiches: heroes, hoagies, grinders, subs. The French have theirs, too: the tartine is an open-faced sandwich on toasted, buttered bread, topped with a variety of ingredients. While slimmer than our bulky bunch, tartines are packed with as much satisfying flavor. This sandwich takes its cue from the classic salade niçoise, where tuna, beans, hard-boiled eggs, tomatoes, olives, and herbs mingle for a crisp meal popular in the French Riviera city for which it's named. Our version might make every other tuna sandwich seem so passé.
In a bowl, combine the tuna, pine nuts, egg, olives, tomato, beans, celery, onion, and parsley. In a small bowl, whisk together the mustard, sugar, and lemon juice. Slowly whisk in the oil until a smooth emulsion forms. Pour the vinaigrette over the tuna mixture and toss gently to coat.
Butter each piece of toast if you wish and top with a few heaping spoonfuls of tuna salad.